This invention relates generally to multiplexing systems, and more particularly to a scintillation monitoring system wherein a predetermined number of light sensors monitor and count scintillation events in a greater number of scintillation samples, or a larger number of monitor points than there are light sensors.
Liquid scintillation counting of low level radioactivity is commonplace in laboratories where pharmacological, biochemical, or molecular biological research is conducted. In addition, such scintillation monitoring systems are found in hospitals and clinics which perform laboratory tests as part of clinical procedures. In a typical scintillation monitoring arrangement, scintillation counters automatically count between one hundred and three hundred sequential samples each for a time period typically between one and twenty minutes. Counting is generally performed for a predetermined period of time, or a predetermined number of counts, or some variation of these two limits. In either case, statistical fluctuations in a given sample are generally required to be lower than five percent. Assuming that Gaussian statistics are applicable to such counting, two thousand disintegrations per sample are implied.
In addition to the need to count scintillation events in sequential samples, there is a need to count scintillation events in a continuous flow process, such as in a chromatography column, without interruption of the flow or batch processing. However, a system for achieving a continuous count of flowing effluent would require a large number of photosensors, depending upon the speed of the flow of the effluent and its radioactivity. Moreover, the photosensors should be arranged in pairs to achieve a reliable count of the scintillation events, while reducing the effects of noise.
It is a problem with known systems that they are expensive to purchase, use, and maintain. For example, typical scintillation counters cost between $15,000.00 and $30,000.00, and generally require approximately $2,000.00 per year to maintain. This requires a high rate of throughput for economic reasons. One approach to the problem of achieving high throughput is to add sufficient radioactivity to the initial reaction to achieve a high count rate in the product which is to be measured. Such highly radioactive samples, however, raise the problem of a significant addition to the cost of the laboratory procedure, possibly offsetting whatever financial gain might be derived from a shorter counting time. In addition, potential environmental hazards are produced in the laboratory as a result of usage of large amounts of radioactivity. It is highly desirable for health reasons to keep the usage of radioactivity to a minimum. In many radiolabeling procedures, only a fraction of a percent of the added radioactivity is incorporated and counted, and the remainder must be disposed of. In addition to the foregoing, it should be noted that the biohazards resulting from use of radioactivity extend beyond the laboratory. For example, long-term storage of low-level radioactive waste is receiving increased public attention. Tritium and .sup.14 C are particular sources of concern since they have half-lives of 12 years and 5,600 years, respectively. The cost of waste disposal has escalated and may eventually become the limiting factor in biomedical research which employs radioactivity.
It is evident from the foregoing that there is a need for a scintillation monitoring system which can monitor the occurrence of scintillation events in a large number of samples simultaneously. Such a system would be particularly useful in biodistribution studies which use new radiotracer drugs, particularly for human applications. Such biodistribution studies are necessary to see how the drug is metabolized and which organ receives the highest radiation dose. The nuclides involved may have half-lives from 2 minutes (.sup.15 O) to 20 minutes (.sup.11 C). With such short half-lives, it is necessary to inject large amounts of material so that by the time the last tissue is counted in a conventional scintillation counter, there will remain sufficient radioactivity for accurate measurement. There is a need for a counting system which will measure all tissues simultaneously thereby obviating the need to correct for decay of the isotopes, since all samples will be decaying constantly, and the percent distribution to the various tissues will not vary during the counting period.
It is, therefore, an object of this invention to provide a simple and inexpensive system which can accommodate a broad range of scintillation sources.
It is another object of this invention to provide a simple and inexpensive scintillation monitoring system which can monitor many scintillation vials simultaneously.
It is a further object of this invention to provide a scintillation monitoring system which requires relatively few sensors, such as photomultiplier tubes, and has a high throughput.
It is an additional object of this invention to provide a scintillation monitoring arrangement which affords a reduced consumption of radioactive materials.
It is still another object of this invention to provide a system for simultaneously counting scintillation events from multiple vials at moderate cost.
It is a still further object of this invention to provide a system which is useful for multiple simultaneous short half-life experiments.
It is also an object of this invention to provide a system for which light collection efficiency can easily be measured.
It is yet another object of this invention to provide a multiplexing system which employs multiple inputs to each of a plurality of sensors and coincidence logic.
It is also another object of this invention to provide a system for monitoring 25 or more radioactive samples using less than two photodetectors per vial.
It is also a further object of this invention to provide a multiplexing sytem which utilizes relatively few sensors to monitor many samples without intolerable cross-talk interference.
It is additionally an object of this invention to provide a system which achieves high efficiency using common beta-emitting radionuclides, such as .sup.3 H, .sup.14 C, .sup.32 P, and .sup.35 S.
It is yet another object of this invention to provide a system which achieves high throughput without requiring high levels of radioactivity.
It is a yet further object of this invention to provide a system which facilitates in vivo biodistribution studies of short-lived isotopes.
It is additionally another object of this invention to provide a system which enables sensing of multiple labels simultaneously.
A still further object of this invention is to provide a scintillation monitoring system which permits continuous flow to be monitored, without interruption.
An additional object of this invention is to provide a scintillation monitoring system which permits a continuous flow path to be monitored at multiple points therealong without requiring a correspondingly large number of sensor pairs.
Yet another object of this invention is to provide a system for monitoring a continuous flow output from a chromatography column whereby the effective time of exposure of a photosensor to a particular quantum, or slug, of the effluent of the column is increased, without slowing the flow.